1. Field of the Invention
The present invention relates generally to a method and apparatus for optical multiplexing and demultiplexing, and more particularly to a compact wavelength optical multiplexer-demultiplexer device providing high spectral resolution and low polarization dependency.
2. Description of the Prior Art
The explosive growth of telecommunication and computer communications, especially in the area of the Internet, has created a dramatic in increase in the volume of worldwide data traffic which has placed an increasing demand for communication networks providing increased bandwidth. To meet this demand, fiber optic (light wave) communication systems have been developed in order to harness the enormous usable bandwidth (tens of tera-Hertz) of a single optical fiber transmission link. Because it is not possible to exploit all of the bandwidth of an optical fiber using a single high capacity channel, wavelength division-multiplexing (WDM) fiber optic systems have been developed to provide transmission of multi-carrier signals over a single optical fiber thereby channelizing the bandwidth of the fiber. In accordance with WDM technology, a plurality of superimposed concurrent signals are transmitted on a single fiber, each signal having a different wavelength. WDM technology takes advantage of the relative ease of signal manipulation in the wavelength, or optical frequency domain, as opposed to the time domain. In WDM networks, optical transmitters and receivers are tuned to transmit and receive on a specific wavelength, and many signals operating on distinct wavelengths share a single fiber.
Wavelength multiplexing devices are commonly used in fiber optic communication systems to generate a single multi-carrier signal, in response to a plurality of concurrent signals having different wavelengths received from associated sources or channels, for transmission via a single fiber. At the receiving end, wavelength demultiplexing devices are commonly used to separate the composite wavelength signal into the several original signals having different wavelengths.
Dense wavelength division multiplexing (DWDM) devices provide multiplexing and demultiplexing functions in specific wavelength ranges. Important design criteria for a DWDM device include a large number of channels, narrow channel spacing, low inter-channel cross talk, low insertion loss, low polarization dependency, compactness, environmental stability, and low manufacturing cost.
U.S. patent application Ser. No. 09/193,289, filed Nov. 17, 1998, entitled xe2x80x9cCOMPACT DOUBLE-PASS WAVELENGTH MULTIPLEXER-DEMULTIPLEXERxe2x80x9d and U.S. patent application Ser. No. 09/362,926, filed Jul. 27, 1999, entitled xe2x80x9cCOMPACT DOUBLE-PASS WAVELENGTH MULTIPLEXER HAVING AN INCREASED NUMBER OF CHANNELSxe2x80x9d have at least one inventor in common with the present application, and are hereby incorporated by reference. Each of these Patent Applications describes a multiplexer/demultiplexer device including: a fiber mounting assembly for aligning an array of optical fibers for transmitting optical signals; collimating and focusing optics (e.g., a lens) for collimating and focusing optical beams; a transmission grating having a diffractive element that provides diffraction of optical beams; and a reflective element such as a mirror. The fiber mounting assembly supports a plurality of close-spaced optical fibers such that ends of the fibers are disposed substantially in a common plane. The collimating and focusing optics, transmission grating, and mirror are designed to provide efficient operation in selected communication wavelength regions.
During operation of one these devices as a demultiplexer, an input optical beam including a plurality of individual wavelengths is transmitted to the device via an input one of the optical fibers, and radiated from the end of the input fiber which is located at the vicinity of a focal point of the collimating lens. Divergence of the radiated input beam depends on the numerical aperture of the input fiber. The lens has a sufficient numerical aperture to accept the diverging beam from the input fiber and substantially collimate the beam which is then passed through the diffractive element causing the wavelengths of the input beam to be diffracted and separated according to their wavelengths. The spatially separated beams are redirected by the mirror back to the diffractive element which provides further spatial separation of the individual wavelengths, thereby enhancing the total dispersion effect. The spatially separated beams are then focused by the focusing lens and received directly by a plurality of output ones of the optical fibers.
During operation of the device as a multiplexer, the beam directions are essentially reversed as compared to the beam directions during operation in the demultiplexer mode. In this embodiment, each of the optical beams provided by the fibers has a different wavelength. The wavelengths of each of the beams are collimated by the lens, and then diffracted by the diffractive element with specific angular orientations according to the specific wavelength of the individual beam. The diffracted beams are reflected by the mirror, diffracted again by the diffractive element, and eventually merged into a substantially collimated beam including all of the wavelengths. This collimated beam is then focused by the lens onto an output one of the optical fibers.
Each of the devices described in U.S. patent application Ser. No. 09/193,292 and U.S. patent application Ser. No. 09/362,926 satisfies most of the important design criteria for operation of the device as a dense wavelength division multiplexer (DWDM) device. However, these devices have certain limitations which pose a difficulty in minimizing the physical size of the device while maximizing the transmission capacity.
In order to increase the transmission capacity of a fiber communication network using a DWDM device of the type described above, it is possible to increase the number of channels by decreasing the channel wavelength spacing while maintaining the physical spacing between the optical fibers in the array. The number of channels provided by a device is proportional to the linear dispersion provided by the device. The linear dispersion of a wavelength separation device of the type described above can be expressed generally in accordance with relationship (1), below.
xcex4L/xcex4xcex=fxc2x7(xcex4L/xcex4xcex8)=(fxc2x7m)/(dxc2x7COS xcex8)xe2x80x83xe2x80x83(1)
Where xcex4L/xcex4xcex8 represents linear dispersion provided by the device, f is the focal length of the collimating and focusing element, and d is the groove spacing of the diffractive element of the grating. Note that the groove spacing d of the diffractive element is inversely proportional to the groove density of the diffractive element.
Relationship (1) suggests two obvious methods of increasing the number of channels in a DWDM device. These methods include: increasing the focal length f of the collimating and focusing element; and increasing the groove density of the diffractive element of the grating. However, these methods of increasing the number of channels in the device are complicated by the fact that modern optical networks demand smaller and smaller physical device sizes while also requiring high optical performances as further explained below.
The first method of increasing the number of channels of the DWDM device includes increasing the focal lens f of the device, thereby increasing the spectral resolution of the device and reducing the channel spectral spacing between adjacent fibers. However, a consequence of increasing the focal lens f of the device is that the over-all physical size of the device will be increased. Increasing the size of the device leads to increased costs of production, and also makes environmental responses (e.g., thermal responses, stress responses, etc.) of the mechanical and optical assembly of the device difficult to control.
The second method of increasing the number of channels includes increasing the grating groove density (thereby reducing the groove spacing d) in order to provide higher dispersion and thus higher spectral resolution. It is possible to reduce the focal length f of the optical device while increasing the groove density in order to minimize the size of the device and still maintain, or even increase the spectral resolution. Maintaining the focal length of the optical element of the device while increasing the groove density of the grating allows for a smaller size device and increased dispersion. However, increasing the groove density of the grating requires an increase in the optimal grating incidence angle. The grating incidence angle of a beam incident on the grating (diffractive element) is defined by the angular difference between the propagation direction of the beam and a vector that is normal to the surface of the grating. A problem of increased polarization sensitivity arises in increasing the groove density of the grating, and increasing the optimal grating incidence angle.
Polarization sensitivity refers to the variation in the spectral resolution of a device as a function of variations in the polarization state of optical beams input to the device. Polarization sensitivity in a multiplexer-demultiplexer device is caused by variations in the diffraction efficiency of the grating of the device as a function of variations in the plane of polarization of an input beam with respect to the orientation of the diffractive element which is defined by the groove distribution direction of the diffractive element. As the groove density of the diffractive element of the grating is increased, and the optimal grating incidence angle is increased, the diffraction efficiency of the grating of the device becomes more dependent on variations in the polarization state of incoming beams which leads to an increase in the overall device polarization sensitivity.
It may be possible to fine tune the grating incidence angle during fabrication of the grating assembly so that the grating efficiencies are similar for both S-polarization and P-polarization light incident on the grating. However, by fine tuning the grating incidence angle in this manner, the absolute value of the grating efficiency is significantly reduced causing the device to have a high insertion loss which is undesirable for a wavelength multiplexer/demultiplexer device.
Minimal polarization sensitivity is an important design parameter for multiplexer demultiplexer devices to be used in optical communication networks because the polarization state of beams transmitted via the network can vary over time due to a wide variety of optical effects in the network, and it is essential that the performance of a multiplexer-demultiplexer device does not vary with the polarization state of beams transmitted thereto. Therefore, what is needed is a compact wavelength multiplexer-demultiplexer device that provides increased spatial resolution while also providing minimal polarization sensitivity,
It is therefore an object of the present invention to provide a dense wavelength division multiplexing (DWDM) device having an increased number of channels and low polarization sensitivity.
It is also an object of the present invention to provide a DWDM device that accommodates large transmission capacity while being small, lightweight, immune to temperature variation and stress-induced instability, and inexpensive to produce.
Another important object of the present invention is to provide DWDM device that is easy to manufacture in large quantities using components that are easy to make and assemble.
Briefly, a presently preferred embodiment of the present invention provides an optical multiplexing and demultiplexing device including: fiber mounting means for securing a plurality of optical fibers each terminating in a fiber end, at least one of the fiber ends radiating a corresponding input beam having a polarization state defined by a polarization plane that may vary as a function of time, and at least one of the fiber ends for receiving an output beam; collimating and focusing means having a focal length, and providing collimation of the input beams, and focusing of the output beams; diffraction means including at least one diffractive element providing for dispersion of optical beams passing therethrough, each diffractive element having an associated diffraction efficiency that varies as a function of the polarization state of light beams passing therethrough; and a polarization rotating means for rotating the plane of polarization of a beam passing therethrough by a specified number of degrees.
The device provides for a particular input beam having a first polarization state to be diffracted by the diffractive elements in accordance with a first diffraction efficiency based on the first polarization state. The beam is then passed through the polarization rotating means at least once and thereby rotated into a second polarization state. The rotated beam is then diffracted by at least one of the diffractive elements in accordance with a second diffraction efficiency based on the second polarization state, whereby the device provides a spectral resolution that is substantially insensitive to variations in the polarization state of the input beam.
In one embodiment, the device further includes a reflective element, and the polarization rotating means is disposed between the reflective element and the diffraction means. Also in this embodiment, the diffraction means includes a transmissive diffractive element. This embodiment of the device provides for a particular input beam having a first polarization state to be transmissively diffracted a first time by the diffraction means in accordance with a first diffraction efficiency based on the first polarization state, passed through the polarization rotating means a first time, reflected by the reflective element back to the polarization rotating means, passed through the polarization rotating means a second time and thereby rotated into a second polarization state, and transmissively diffracted a second time by the diffraction means in accordance with a second diffraction efficiency based on the second polarization state.
In one embodiment, the polarization rotating means includes a magnetic-optic crystal. The magnetic strength and thickness of the crystal may be selected to provide a selected rotation of the plane of polarization of beams passing therethrough. In one embodiment, the polarization rotating means provides for rotating the plane of polarization of a beam passing once therethrough by approximately 45 degrees.
In another embodiment, the collimating and focusing means includes an off-axis parabolic mirror. This embodiment of the device may be formed using individual parts or may be formed as an integral unit. In an embodiment wherein the device is formed as an integral unit, a chamber means is formed, the chamber having a first wall forming a parabolic surface at least a portion of which is polished to provide the off-axis parabolic mirror, and a second wall having a planar surface at least a portion of which is polished to provide the reflective element. In one embodiment, the polarization rotating means is formed by a crystal having at least one planar surface, and the planar surface of the crystal is contiguous with the polished portion of the second wall.
A single grating pass embodiment of the device includes: first and second fiber mounting assemblies forming the fiber mounting means, the first fiber mounting assembly for securing a single optical fiber terminating in a fiber end, the second fiber mounting assembly for securing an array of secondary optical fibers each terminating in a fiber end; a first lens and a second lens providing the collimating and focusing means, the first lens for focusing and collimating beams propagating between the ends of the single fiber and the diffraction means, the second lens for focusing and collimating beams propagating between the ends of the array of optical fibers and the diffraction means; a first reflective element for reflecting beams radiating to and from the single fiber via the diffraction means, and for reflecting beams radiating to and from the polarization rotating means; and a second reflective element for reflecting beams radiating to and from the array of optical fibers via the diffraction means, and for reflecting beams radiating to and from the polarization rotating means. In this embodiment, the polarization rotating means includes a single optical active crystal designed to provide for rotating the plane of polarization of a beam passing once therethrough by approximately 90 degrees. This embodiment of the device may also be formed as an integral unit including means forming a chamber having a first wall at least a portion of which is polished to provide the first reflective surface, and a second wall having a planar surface at least a portion of which is polished to provide the second reflective surface.
Another single grating pass embodiment of the device includes: first and second fiber mounting assemblies forming the fiber mounting means, the first fiber mounting assembly for securing a single optical fiber terminating in a fiber end, the second fiber mounting assembly for securing an array of secondary optical fibers each terminating in a fiber end; first and second transmissive diffractive elements forming the diffraction means, the polarization rotating means being disposed between the first and second diffractive elements; and a first lens and a second lens providing the collimating and focusing means, the first lens for focusing and collimating beams propagating between the end of the single fiber and the first diffractive element, the second lens for focusing and collimating beams propagating between the ends of the array of optical fibers and the second diffractive element. During operation of this device in a demultiplexing mode, a particular input beam having a first polarization state is transmissively diffracted a first time by the first diffractive element in accordance with a first diffraction efficiency based on the first polarization state, passed through the polarization rotating means and thereby rotated into a second polarization state, transmissively diffracted a second time by the second diffractive element in accordance with a second diffraction efficiency based on the second polarization state, and focused by the second lens.
In yet another embodiment of the device of the present invention, the diffraction means and the polarization rotating means are both provided by a dual grating assembly including a first diffractive element having a first grating orientation defined by a first groove distribution direction, and a second diffractive element having a second grating orientation defined by a second groove distribution direction, the first and second groove distribution directions being substantially orthogonal to each other. The first and second diffractive elements are substantially identical, each having a substantially equal groove density and providing a substantially equal diffraction efficiency for incident beams having a plane of polarization that is oriented at the same angle relative to the groove distribution direction of the diffractive element. However, the diffraction efficiency provided by each of the first and second diffractive elements varies as a function of the orientation of the plane of polarization of incident beams relative to the groove distribution direction of the diffractive element. However, because the first and second diffractive elements are oriented so that the first and second groove distribution directions are substantially orthogonal, a beam passing through both the first and second diffractive elements is diffracted in accordance with a total diffraction efficiency that is independent of the polarization state of the incident beam, that is the orientation of the plane of polarization of the incident beam.
An important advantage of the device of the present invention is that it is substantially insensitive to variations in the plane of polarization of incoming optical signals.
Other advantages of a device of the present invention include small size, light weight, and immunity to environmental stress.